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ELECTRICAL PIEZOMETERS User Manual Electrical Piezometers INDEX Introduction Pag. 4 Description Pag. 5 Filters saturation Pag. 6 Preliminary checks Pag. 7 Installation Multipoint piezometer Drive-in piezometer Removable piezometer Pag. 8 Pag. 10 Pag. 12 Pag. 12 Taking measurements Pag. 13 Data management Pag. 13 Troubleshooting Pag. 15 Maintenance Pag. 15 Appendix 1 Pag. 16 Information contained herein are property of SISGEO S.r.l. This document is subject to change without notification and is subject to be returned upon request. No part of this User’s Manual may be reproduced in any form without SISGEO’s S.r.l. written permission. SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 2 of 16 Electrical Piezometers Notes on the use of product For a safe and efficient use of the instrument, please read carefully the following instructions before starting any operation. Any use of the instrument other then the one described in this manual shall be considered at user’s full responsibility. The same applies for any unauthorized modifications. In addition to the hereby listed standards, the user must comply with the provisions of the current legislation on the matter of personal safety and health of persons in the workplace. SISGEO is not responsible for any trouble, breakdowns, accidents etc.. due to the lack of knowledge and/or confidence (or non-compliance with) with the requirements contained in this manual. Check that the instrument has not been damaged during the transport. Verify that the package includes all items as well as any requested optional accessories; if anything is missing, please promptly contact the manufacturer. The user must strictly follow all the operations described in this manual. Maintenance or repair of the instrument is allowed only to authorized operators. These operators must be physically and intellectually suitable. For information about instrument or order spare parts request, please always specify data written on the identification label. When replacing parts, always use ORIGINAL SPARE PARTS. The manufacturer reserves the right to make changes without prior notice for any technical or commercial requests. We’ll try anyway to keep the manuals updated in order to reflect product’s revisions/updates. Symbols This symbol will be used used to catch reader’s attention on the manual: Pay special attention to the following instruction. Identification SISGEO S.r.l. Instruments can be identified • From a production lot number (written on the Compliance Certificate) • From a serial number (s/n) engraved indelibly on the instrument • From a label on the instrument • From a label on the cable Electrical piezometers_EN_ 01_13 Page 3 of 16 Electrical Piezometers Introduction Measure panel Electrical piezometers are largely used in civil and foundations engineering, to monitor groundwater level and to measure pore water pressure. Electrical piezometers, called “closed circuit type”, are installed in borehole, embankments or directly by drive-in if the ground features allow the operation. Support head assembly Bentonite How it works: the membrane deforms due to water pressure, causing the variation of an electric signal, measured with a measuring device. SISGEO uses two kind of sensors: Piezometer • Vibrating wire • Resistive In the vibrating wire piezometers (VW) the membrane deformation causes the tension variation of a steel wire stretched between the membrane and the instrument body. The vibration frequency is collected from a measuring device. Electric cable Sand Piezometer Bentonite SISGEO S.r.l. This instrument, moreover, includes a thermistor to measure the temperature. The resistive piezometers have a ceramic membrane with molded staringauge that change their resistance and the electric signal according to membrane deformations due to water pressure. The VW piezometers assure an high reliability for long term measures; the resistive allow high insulation (for example in industrial plants) and the possibility to take dynamic measures. Electrical piezometers_EN_ 01_13 Page 4 of 16 Electrical Piezometers Description Electric piezometer consists in: 1. Stainless steel cylindrical body 2. Hydraulic chamber 3. Filter holder and filter 4. Measuring sensor (membrane) 5. Thermistor (only with VW type) 6. Electric cable for the connection with the readout 7. Conical filter tip (drive-in type) 8. Piezometer push-in rod (drive-in type) 9. Conical filter tip (removable type) 1 6 4 2 5 3 8 9 7 The filter is an important element and its choice depends on the use and the ground type. The sintered stainless steel or vjon filters have a porosity of ca. 40µm, are called “Low Air Entry” (LAE) and are used for most standard applications. Their saturation is performed easily on site. Ceramic filters have a low porosity and shall keep the saturation of the hydraulic chamber also if submitted to negative pressures (suction). They are employed when the piezometer is in “dry conditions” for a long time. The ceramic filters are called “High Air Entry” (HAE). The saturation of this kind of filters is not easy and has to be done with a suitable device. SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 5 of 16 Electrical Piezometers Filters saturation Filters saturation and assembly The purpose of filters saturation is to remove the air in filters pores and replace it with water in order to have a faster answer to pressure variations and more accurate measures; moreover it prevents ground particles to obstruct filter pores. The air entry is the pressure necessary to force the air through a porous filter completely saturated. This value is commonly used in geotechnical field and it is proportional both to pores diameter and the fluid used for the saturation. Typical values of “low air entry” are the ones between 3 and 30KPa while typical values of ”high air entry” are the ones higher than 100KPa. SISGEO recommends to perform the saturation on site just before the installation. LAE Filters Usually the filter must be left in a bucket full of water for at least 24 hours. The saturation is better if the water has been previously de-aired bringing it to boil for at least 10 minutes and then left to cool. Take a bucket with clean water then: • submerge the filter holder with the filter; • submerge the piezometer turned up and fill the hydraulic chamber; • screw the filter holder on the piezometer, working underwater. Follow the same operations for the assembly in case of saturated filters supplied by Sisgeo. HAE Filters using SISGEO saturation device • • • • • • Remove the air from the chosen fluid, usually distilled water, bringing it to boil or vacuum until the air bubbles will disappear; fill in the saturation device; screw the filter holder with the filter on the saturation device; screw slowly the saturation device, and handle it until you could read (on the gauge) the value of ca. 3.5Bar; wait until filter’s surface is not covered with fluid’s drops; unscrew the filter holder and screw it on the piezometer working underwater in a bucket. In case of HAE filters and for instruments with low full scale (<200kPa), is necessary, during the assembly, to connect the piezometer to a readout to check the induced pressure and screw slowly the holder on the piezometer in order to avoid overpressures that could damage it. SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 6 of 16 Electrical Piezometers Filter saturation and assembly in SLIM piezometers (cod. PK20xxx) For the saturation follow the instruction in the paragraph of LAE filters. For the assembly, push and turn the holder on the piezometer body. In case of HAE filters, the operation could take a few minutes. The saturation is successful when the liquid leaks from the filter surface. In case of HAE filters and for instruments with low full scale (<200kPa), is necessary, during the assembly, to connect the piezometer to a readout to check the induced pressure and screw slowly the holder on the piezometer in order to avoid overpressures that could damage it. Preliminary checks Before the installation is recommended to check the instrument connecting it to a readout (see “Taking measurements”). The read value has to be similar to the “zero value” written on the Calibration Report. The possible differences could be due to height, barometric pressure and, for the VW type, temperature. This operation is possible with saturated or unsaturated filter. Please remember that the Calibration Report shows the conditions during the test, and it is issued at 115 m. s.l. The atmospheric pressure lowers approximately 0.1kPa every 10m height. We advise to leave the VW model temperature to stabilize for at least 20 minutes before taking any measure. The acceptable differences between the measured value and the one shown on the Calibration Certificate with equal conditions are: • Extensometer model :±1% of full scale =0.16mA • VW model: ±100digits For low full scale values, is necessary to consider the barometric pressure value. Indeed, on a transducer of 0,2MPa a barometric pressure variation of 50mbar (0,005MPa) is like 2,5% of full scale (1 millibar is 1cm of water level). SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 7 of 16 Electrical Piezometers Installation Installation in boreholes Installation procedures must be defined according to the grounds and the purpose of the measures. In general terms you can use the following procedure: Boreholes can be executed with continuous core or non-core drilling. We always recommend to steady the walls with a temporary casing, with an internal diameter not less than 85mm (1 piezometer for borehole), 110mm (2 piezometers for borehole). Avoid the use of mud. To install two piezometers in the same borehole are needed two filters and two caps: pay attention to stratigraphy and thickness of layers. SISGEO recommends to install maximum two piezometers for each borehole. To stabilize the bottom of the hole, without ground water, water level must be kept within the drilling, a little above the ground level. This solution has to be used also during the installation steps. Tools needed during installation: depth meter, pestle, bentonite pellets, bucket. To install, proceed as shown in the picture: Bentonite sealing Lining casing ~100cm ~50cm ~70cm Piezometer Sand SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 8 of 16 Electrical Piezometers Suggestions: • Identify the installation depth, on piezometer cable, starting from the membrane position engraved on the steel body. • Before, through and after the installation check piezometer measurement. • Wash the borehole until it leaks clear water. • Fill in the geotextile bag with sand and insert the piezometer with the saturated filter; then put everything in a plastic bag filled with water and dip it in the casing full of water. Now break the plastic bag, pull out the piezometer and drop it slowly. These operations must be carried out always underwater. • ∅ fine sand or gravel = ca. 2mm. • Carry out the bentonite cap in several times (ca. 25÷30cm each time). Compact with a pestle. Pay attention to the cable. • Verify cap solidity and depth with a depth meter. • Approximate quantities for the grout mixture: 100l of water, 50Kg of concrete and 5Kg of bentonite • Cable shall always be covered to avoid casual damages and protect the end from wet. With an higher impermeable soil layer, the bentonite sealing should be carried out at the same level of the layer in order to restore the continuity. SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 9 of 16 Electrical Piezometers Multipoint Piezometer Lately, as a result of studies, is taking over a new installation method called “fully grouted”. This method is very easy and fast. It doesn’t expect to use gravel and bentonite to create filter and sealed zones, but a mixture of water-cement-bentonite inside the borehole, with the right proportions according to the installation. The mixture is injected straight to the drilling. This allows to install more piezometers in the same hole, and also to install the piezometers in combination with an inclinometer casing. Approximately the proportions (in weight) of the grout mixture are: cement 1, water 2.5, bentonite 0.3 We recommend to mix in advance water and cement, and then add the bentonite. Measures can be taken only after 28 days. This kind of installation is suitable with multipoint piezometers, where is possible to install up to 8 piezometers in the same borehole. P1 P2 P3 SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 10 of 16 Electrical Piezometers Installation in fills and embankments Piezometers can be installed in embankments or in core dams to measure the pore water pressure through the construction and working. Pores of compacted unsaturated material contains both water and air and in finegrained soil, the pressure difference between the pore water pressure and pore air pressure can be significant. For clay embankments, piezometers must have an HAE filter, to ensure pore water pressure measure, and not air. In granular materials as sand, gravel and rock fill you can use a LAE filter. To install proceed as follows: Piezometer Filling material compacted by hand • • • • • • • • • • • • • SISGEO S.r.l. Cable Before, through and after the installation check the piezometer Dig a trench ca. 50 cm. width and 30-40 cm. deep. Saturate the piezometer as previously described. For installation in coarse material: insert the piezometer in a geotextile bag filled with sand. If the piezometer has an HAE filter and has to be installed in an unsaturated ground, assemble only the filter without using the geotextile bag. In coarse ground, dig a hole and insert the piezometer. Close the hole with the filling material compacting manually. In case of an embankment with big size materials, we recommend to cover the hole with saturated sand compacted by hands; For installation in unsaturated grounds, fix the piezometer in trench wall or dig a little hole and fix the piezometer; Lay out the electric cable within the trench in a snake-like mode, to avoid damages during the embankment settlement; with coarse ground, protect the cable with a conduit. Cover the cable with fine material in consecutive layers of 5-10 cm each and compact by hands. Arrange some bentonite pellets in regular intervals along the cable, in order to avoid water leaks and compact manually. Mark piezometer position and protect cable end from water seepage. Electrical piezometers_EN_ 01_13 Page 11 of 16 Electrical Piezometers Drive-in piezometer installation Drive-in piezometer, both ceramic and vibrating wire, is a special version suitable to be pushed directly in soft grounds. The drilling equipment must tighten the push-in rods on the perimeter. Otherwise it will be necessary to have a rod with a groove for the cable. To install proceed as follows: • Arrange the instrument according to usual procedures; • Protect the end of the cable with some tape and insert the first push-in rod assembled on the first rod. The installation can be started from the bottom of the borehole or from the ground level. Well Mixer Drive-in piezometer Bentonite Push-in rod • • • • • • Concrete-bentonite mixture For installation from borehole bottom, lower, in the hole, the push-in rods with the piezometer. Connect the piezometer to the readout and check non-stop the overpressure values resulting from the push, to avoid damages to the sensor: if necessary stop and wait for the excessive pressure to disperse. Push in until the chosen depth. Disconnect the piezometer; protect cables end with some tape and pull out the rods avoiding any stretch on the electric cable. Connect the piezometer and check the good functioning. Close the hole with bentonite balls or a mixture of water, concrete and bentonite. Wait until the values are stable (and the overpressure is dispersed) to take the zero reading. Removable piezometer with conic filter tip installation SISGEO S.r.l. • Lower the piezometer and the weights within the pipe using the kevlar wire within the electric cable. • Insert the piezometer in the Casagrande filter (previously installed). • If the Casagrande filter is double tube, you must lower, in the ½ ” tube, the steel rod with conic cap and O-Ring, using the steel wire. Electrical piezometers_EN_ 01_13 Page 12 of 16 Electrical Piezometers Taking measurements Manual readings are taken connecting the conductors to a readout according to the following scheme: Resistive piezometers Red + Loop Signal 4-20mA current loop Black - Loop Red VW Vibrating wire piezometers (VW) Black VW White Thermistor Green Thermistor For the multipoint piezometers From 1 to 4 piezometers using a multicore cable code 0WE1160000: VW VW Thermistor Thermistor P1 Brown White/Brown Green White/Green P2 Yellow White/Yellow Grey White/Grey P3 Blue White/Blue Rose White/Rose P4 Red White/Red Black White/Black From 5 to 8 piezometers using a multicore cable code 0WE1320000: VW VW Thermistor Thermistor P1 Brown White/Brown Black Rose/Black P2 Yellow White/Yellow Grey White/Grey P3 Black Blue/Black Rose White/Rose P4 Red White/Red Black White/Black P5 Violet White/Violet Black Yellow/Black P6 Orange White/Orange Black Red/Black P7 Blue White/Blue Green White/Green P8 Light Blue White/Light blue White Green/White Do not split the conductors to avoid confusion with the colors. Note: to obtain reliable measures, with mA piezometers, we recommend a warm up time not less than 10 seconds. For automatic measures, connect the instrument to a datalogger. SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 13 of 16 Electrical Piezometers Data management The following formulas allow to convert the electrical measures into engineering values: Linear factor Leng = Lele/S [kPa] Polynomial factors Leng = (Lele2 x A) + (Lele x B) + C [kPa] Leng = engineering reading Lelec = electric reading S = sensitivity factor A, B, C = polynomial conversion factors S, A, B, C factors are stated on DTE Calibration Report The exercise readings refer to the initial reading (zero reading). ∆PkPa = Li - L0 L0 = Zero reading [kPa] Li = Exercise reading [kPa] Zero reading shall be taken carefully once the installation is performed and the instrument is in operating conditions. For many applications is necessary to wait a few days to obtain a reliable zero value. Example Piezometer 700kPa (digit readings) S = -4.9128digit/kPa A = -5.222e-07; B =-1.955e-01; C =1.886e+03 L0 = 7357digit ; L1 = 6667digit Using: Linear factor (L1-L0)/S : (6667 – 7357)/-4.9128 = 140.4494 Polynomial factor [(L12 x A)+(L1x B)+C] – [(L02 x A)+(L0x B)+C]= 559.39 – 419.44 = 139.95kPa Temperature measure Using a SISGEO readout, the temperature is read directly in C°. If measured, the thermistor resistance value must be converted using the formula or the table shown in appendix 1. If the piezometer is installed in an environment with uneven temperature, is necessary to correct pressure value already processed using the follow equation: PT = (Ti - T0) K where: PT = correct pressure Ti = excercise temperature; T0 = zero temperature; K = temperature factor (can be found on the piezometer calibration certificate) Add the obtained correction to the pressure value found with the previous formula. Ptot = (P + PT) Example K= 0.154344kPa/°C Ti=28°C ; T0=15°C PT=(28-15) x 0.154344 = 2.006kPa Ptot = 139.95+2.006 = 141.956kPa SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 14 of 16 Electrical Piezometers Troubleshooting Considering its special application, eventual malfunctioning have to be found along the cable. Vibrating wire piezometers: Problem Possible cause Solution Unstable measure Cable shield not connected Connect the shield Electromagnetic fields generated by engines, generator, antennas, welders or high voltage lines nearby Grounding not well done Identify and remove the cause. Shield the signal cable. Cable cut or damaged. Measure the resistance between conductor Red and Black (150Ω ± 10%). Please consider cable length. The conductor resistance for model 0WE116000 is ca. 88 Ω/km Wiring not correct Repair the cable. Cable splicing kit available at SISGEO. Wire not detected Provide efficient grounding Make proper wiring Resistive piezometers Maintenance Problem Possible cause Solution Unstable measure 0mA measure Overrange measure Wiring not correct Make proper wiring Cable cut or damaged Repair the cable. Cable splicing kit available at SISGEO. After-sales assistance for calibrations, maintenance and repairs, is performed by SISGEO’s service department. The authorization of shipment shall be activated by RMA “Return Manufacturer Authorization". Fill in the RMA module clicking on: http://www.sisgeo.com/en/assistance/repairs/ Send back the instrument/equipment with the complete accessories, using suitable packaging, or, even better, the original ones. The shipping costs shall be covered by the sender. Please return to the following address with suitable delivery document: SISGEO S.r.l. Via F.Serpero, 4/F1 20060 MASATE (MI) On the delivery document is mandatory to indicate the RMA code received. Technical assistance e-mail: [email protected] SISGEO S.r.l. Electrical piezometers_EN_ 01_13 Page 15 of 16 Electrical Piezometers Appendix 1 THERMISTOR TEMPERATURE CONVERSION Resistance to temperature equation: T= 1 − 273.2 3 A + B (LnR ) + C (LnR ) Where: T= temperature in °C LnR= natural Log of the thermistor resistance A= 1.4051x10-3 (coefficents calculated over the -50 to +70°C span) B= 2.369x10-4 C=1.019x10-7 SISGEO S.r.l. Ohms Temp Ohms Temp Ohms Temp Ohms Temp 16.60K -10 5971 10 2417 30 1081 50 15.72K -9 5692 11 2317 31 1040 51 14.90K -8 5427 12 2221 32 1002 52 14.12K -7 5177 13 2130 33 965.0 53 13.39K -6 4939 14 2042 34 929.6 54 12.70K -5 4714 15 1959 35 895.8 55 12.05K -4 4500 16 1880 36 863.3 56 11.44K -3 4297 17 1805 37 832.2 57 10.86K -2 4105 18 1733 38 802.3 58 10.31K -1 3922 19 1664 39 773.7 59 9796 0 3784 20 1598 40 746.3 60 9310 -1 3583 21 1535 41 719.9 61 8851 2 3426 22 1475 42 694.7 62 8417 3 3277 23 1418 43 670.4 63 8006 4 3135 24 1363 44 647.1 64 7618 5 3000 25 1310 45 624.7 65 7252 6 2872 26 1260 46 603.3 66 6905 7 2750 27 1212 47 582.6 67 6576 8 2633 28 1167 48 562.8 68 6265 9 2523 29 1123 49 543.7 69 525.4 70 Electrical piezometers_EN_ 01_13 Page 16 of 16